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Texas Instruments Voltage Supervisors (Reset ICs): Frequently Asked Questions (FAQs) Application notes
Application Report
SLVAE47 – October 2018
Voltage Supervisors (Reset ICs): Frequently Asked
Questions (FAQs)
Michael DeSando
ABSTRACT
This Application Note answers the most commonly asked questions for Voltage Supervisors/Reset ICs,
Voltage Detectors, Watchdog Timers, and all related monitoring devices.
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Difference Between Voltage Detector and Voltage Supervisor?
How to Add Programmable Delay to Voltage Detectors?
How to Create a Fault Detector Using Any Type of Sensor?
How to Adjust the Trigger Voltage on Fixed Threshold Devices?
How to Extend the Input Voltage Range of a Voltage Supervisor?
How to Calculate the Total System Quiescent Current (Iq) for Various Types of Voltage
Supervisors/Detectors?
What is the Function of a Watchdog?
Difference Between "Standard" and "Window" Watchdog?
How to Ultilize a Voltage Supervisor to Enable/Disable a Buck Converter?
What is Device Accuracy? Device Tolerance? How to Calculate Worst-Case Detection?
Mitigating the Indeterminate Output of a Voltage Supervisor (Reset IC) During Power-Up/Down?
What is the Minimum Pulse Width to Cause a Reset?
How to Latch a Voltage Supervisor? How to Latch a Watchdog Timer?
How to Calculate Pull-Up Resistor Value for Active-Low, Open-Drain Devices?
How to Add Hysteresis to a Voltage Supervisor Detection Threshold?
What are Back-Up Battery Voltage Supervisors and Why are they Needed?
Summary
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1
Difference Between Voltage Detector and Voltage Supervisor?
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Difference Between Voltage Detector and Voltage Supervisor?
The main difference between voltage detectors and voltage supervisors is rising output timing, specifically
called the reset delay. When the voltage detector goes from an undervoltage to a normal voltage
condition, the fault is released immediately with a propagation delay typically in the range of
microseconds. Voltage supervisors, on the other hand, have a longer delay in releasing a fault event,
typically in the range of milliseconds.
To be more specific, both voltage detectors and voltage supervisors detect an undervoltage condition and
trigger a fault immediately as shown by tpd(HL) in the figure below. However, the only difference is that
voltage detectors do not have a long delay in releasing that fault event as shown by td(RST) being longer
than tpd(LH).
Voltage supervisors come in both fixed and programmable delays. This delay can be useful to prevent a
false trigger as the output can remain in the fault state until the normal voltage condition has been
detected for a certain period of time.
Figure 1. Voltage Detector (Left) vs Voltage Supervisor (Right)
2
How to Add Programmable Delay to Voltage Detectors?
Voltage detectors usually do not have a reset delay other than the inherent propagation delay. In this
case, a delay capacitor can be added to the output of the open-drain voltage detector to increase the time
it will take for the fault event to release after the device goes from an undervoltage to a normal voltage
condition. Adding a buffer after the capacitor will clean up the output signal, producing a sharp digital
output. The delay capacitor and buffer are shown in the red dotted line in Figure 2.
Vs
Vpull-up
Vmon
VDD
Ipull-up
Voltage Detector
R1
SENSE
R2
System OUT
Rpull-up
(Active-low, Open-drain)
OUT
Logic
Cd
GND
Figure 2. Adding Programmable Reset Delay to Voltage Detectors Circuit Schematic
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The effect of the capacitor and buffer can be compared to the standard configuration without the delay
circuitry shown in Figure 3.
SENSE
V
IT+
VIT-
OUT (no capacitor)
t
tpd(LH)
pd(HL)
OUT (with capacitor)
tpd(LH) + capacitor delay
tpd(HL)
System OUT (after buffer)
t pd(HL)
t pd(LH) + capacitor delay + buffer delay
Figure 3. Adding Programmable Reset Delay to Voltage Detectors Timing Diagram
This can be useful for voltage detectors that have a unique wide voltage input (VIN) specification making
them a very suitable device for applications that only have high voltage rails in the range of 7V or more.
These wide VIN voltage detectors can accept up to 36V at VDD. There may be applications that require a
wide VIN voltage detector but also require a certain reset delay that voltage detectors don’t normally
provide.
Please see Adding Programmable Reset Time Delay to Detectors Application Report for more information.
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How to Create a Fault Detector Using Any Type of Sensor?
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How to Create a Fault Detector Using Any Type of Sensor?
Did you know that voltage supervisors can act as fault detectors for any sensor? By having a sensor that
converts the input measurement to an output voltage, the voltage supervisor or voltage detector can then
be attached to monitor the sensor’s output voltage. A fault can then be triggered by the supervisor when
the sensor reaches a certain threshold. This is useful for detecting over or under fault conditions, as long
as the output voltage range of the sensor and the detect voltage threshold are correctly chosen. An
example of a current monitoring solution is shown in Figure 4.
Vs
Input Current
VDD
IN+
IN-
Rsense
VDD
Vout
GND
RESET
SENSE
output voltage
Fault
GND
Load
output voltage
input curent
Current Sensor
Voltage Supervisor
Figure 4. Using a Voltage Supervisor for Current Sense Fault Protection
The above solution example can be applied to any sensor with a voltage output allowing users to monitor
any sensor and set the fault trigger threshold to any value within the operating range of the voltage
supervisor.
4
How to Adjust the Trigger Voltage on Fixed Threshold Devices?
The TPS3839 is a family of ultra-low quiescent current supply voltage supervisors (SVS) that monitor a
single voltage rail. Because this device is a fixed-voltage monitor, designers must implement different
voltage versions of the TPS3839 for specific voltage rails in their system. However, its low quiescent
current of 150nA makes it suitable for use as an adjustable SVS.
A simple solution for making an adjustable SVS is by adding a resistor divider at the VDD input of the
TPS3839. By setting the resistor divider, the voltage at which to trigger can be varied for different voltage
rails. In addition, this allows for higher voltage rails to be sensed without exceeding the absolute maximum
ratings of the IC.
Please see Using the TPS3839/1 as an Adjustable Supply Voltage Supervisor TechNote for more detailed
information:
4
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How to Extend the Input Voltage Range of a Voltage Supervisor?
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How to Extend the Input Voltage Range of a Voltage Supervisor?
To increase the input voltage range allowed to power a voltage supervisor without damaging the device, a
Shunt Voltage Reference or Zener Diode can be added to the input pin to protect the input pin from going
above the recommend operating input voltage. This solution, shown in the Figure 5, allows a voltage
supervisor to be used at a higher input voltage than the device’s recommended operating range. The
shunt voltage reference variant and the resistor value need to be chosen such that the voltage at Vdd
remains within the recommended operation conditions of the voltage supervisor/detector.
Rs
VREF
Vs > 6.5V
VREF ” 9DD (min)
LM4040
VDD
RESET
Fault
GND
Figure 5. Adding a Shunt Voltage Reference to Increase the Input Voltage Range
The Zener diode and shunt voltage reference only sink current when the supply voltage is above the
breakdown voltage or voltage threshold of those devices. Therefore, when the voltage supply drops below
that breakdown voltage or voltage threshold, the voltage supervisor will operate normally as if there were
no Zener diode or shunt voltage reference in the system.
Please see Using Voltage Supervisors in High Voltage Applications
6
How to Calculate the Total System Quiescent Current (Iq) for Various Types of
Voltage Supervisors/Detectors?
There are two kinds of output topologies, open-drain and push-pull, that must be considered when
accounting for total system current consumption. In an open-drain topology, the supervisor’s Iq, or
quiescent current, does not include the current through the pull-up resistor. If the pull-up voltage is pulled
from the supply, the total system quiescent will increase as supply current will also go through the pull-up
resistor as shown in Figure 6.
Vs
VDD
Ipull-up
Iq (device)
Voltage Supervisor
R pull-up
(Open-drain, no sense)
RESET
Logic
GND
Total system Iq = Iq (device) + Ipull-up
Figure 6. Open-Drain, No Sense Pin, Voltage Supervisor
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How to Calculate the Total System Quiescent Current (Iq) for Various Types of Voltage Supervisors/Detectors?
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If the pull-up voltage is connected to another source, the system Iq will equal the supervisor Iq from the
datasheet in addition to the supply current from the second power source. Also, if the voltage supervisor
has a SENSE pin that is connected to VDD directly or through a resistor divider, the current through the
resistor divider must also be accounted for when calculating total system Iq as shown in Figure 7.
Vs
ISENSE
IRESET
Iq (device)
VDD
Voltage Supervisor
SENSE
R pull-up
(Open-drain, sense pin)
RESET
Logic
GND
Total system Iq = Iq (device) + IRESET + ISENSE
Figure 7. Open-Drain, Sense Pin, Voltage Supervisor
Since the pull-up voltage can be connected to different supplies, the Iq spec of the supervisor does not
account for the additional output current required from the pull-up resistor of the open-drain output
topology.
In a push-pull topology, the total system Iq calculation is slightly different to that of the open-drain. Since
there is no external pull-up resistor is required, the output will not consume additional current as shown in
Figure 8.
Vs
VDD
Iq (device)
Voltage Supervisor
(Push-pull, no sense pin)
VDD
Logic
RESET
GND
Total system Iq = Iq (device)
Figure 8. Push-Pull, No Sense Pin, Voltage Supervisor
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What is the Function of a Watchdog?
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However, similar to that of the open-drain, if a resistor divider is used at the SENSE pin, the total system
current consumption will increase as shown in Figure 9
Vs
ISENSE
Iq (device)
VDD
Voltage Supervisor
(Push-pull, no sense pin)
SENSE
VDD
Logic
RESET
GND
Total system Iq = Iq (device) + ISENSE
Figure 9. Push-Pull, Sense Pin, Voltage Supervisor
Lastly, Figure 10 shows there is no additional Iq when using a level-shifting resistor divider at the output of
a push-pull voltage supervisor.
Vs
ISENSE
VDD
Iq (device)
Voltage Supervisor
SENSE
(Push-pull, no sense pin)
VDD
Iq (device)
Logic
RESET
Level-shifted RESET
GND
Total system Iq = Iq (device) + ISENSE
Figure 10. Push-Pull, Sense Pin, Level-Shifted Output, Voltage Supervisor
7
What is the Function of a Watchdog?
Coupling a voltage supervisor with a watchdog timer allows for both supply voltage monitoring and internal
fault detection of the microprocessor. The watchdog timer adds critical redundancy to systems where a
processor may freeze or hang. Watchdog timers are used for safety-critical applications or for processor
monitoring that requires a reset if the microprocessor is not active for a certain duration. This feature
prevents the system from continuing to run if the microprocessor is not functioning properly.
Voltage detectors and supervisors/reset ICs with watchdog timers wait for signal activity on the watchdog
input (WDI) pin. A reset triggers if the supervisor does not detect a signal within the watchdog window.
You can program this window using an external capacitor, making the watchdog window more flexible.
TI also offers standalone watchdog timers if voltage monitoring is not required: TPS3430/TPS3431
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Difference Between "Standard" and "Window" Watchdog?
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Difference Between "Standard" and "Window" Watchdog?
The difference between “standard” and “window” watchdogs is the lower boundary specification. In
standard watchdogs, a pulse must arrive before the watchdog timeout otherwise a fault occurs. Looking at
the left of Figure 11, notice that the standard watchdog only alerts the system on late faults. In window
watchdogs, a pulse must arrive before the watchdog timeout and after the lower boundary window
otherwise a fault occurs. Taking a look at the right of Figure 11, notice how the window watchdog alerts
the system on both early and late faults.
WDI
Early Fault
WDO
Correct Operation
Correct Operation
WDI
WDI
WDO
WDO
WDI
Late Fault
Late Fault
WDI
WDO
WDO
Figure 11. Standard (Left) vs Window (Right) Watchdog Timers
9
How to Utilize a Voltage Supervisor to Enable/Disable a Buck Converter?
In most applications, the MCU, ADC, or other signal processing device cannot work off an unregulated
supply. Therefore, designers typically use voltage supervisors or detectors to enable or disable the point
of load power management devices. Take the TIDUCR4A, a TI Design for automotive camera’s, as an
example. As shown in Figure 12 the TPS3808 reset output is connected to the PDB pin of the serializer. It
will then monitor the 1.8V rails, as shown in the figure below. The TPS3808 holds the PDB pin of the
serializer in the low state, preventing the serializer from turning on, until the 1.8-V supply voltage rail
reaches a threshold voltage of 1.67 V in this case. Once this threshold voltage is reached, there is a hard
20-ms delay until the supervisor releases the serializer from reset.
Oscillator
FPD-Link III
Serializer
DS90UB933
Video
I2C
CMOS
Imager
AR0140AT
Aptina
1.8 V
Coaxial Cable
PDB
Supervisor
TPS3808G01
Low Vin DC/
DC Buck
TPS62231
2.8 V
RC
Filter
2.9 V
Filter
DC/DC
Buck
TPS62170
Figure 12. Cameria Block Diagram with Voltage Supervisor Protection
8
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What is Device Accuracy? Device Tolerance? How to Calaculate Worst-Case Detection?
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What is Device Accuracy? Device Tolerance? How to Calaculate Worst-Case Detection?
Device accuracy is sometimes confused with device tolerance but these specs are not the same.
Device accuracy refers to the minimum and maximum limits on the voltage threshold. This spec includes
all device variation across all temperature and operating conditions. Sometimes the minimum and
maximum values are written as a number that can be used to calculate the accuracy, as shown in the
chart below.
The calculations for minimum and maximum accuracy are given in Equation 1 and Equation 2:
(1)
(2)
Note: the voltage threshold (VIT) can be displayed in the device datasheet as a voltage value or as a
percent from the nominal VIT.
On the other hand, device tolerance refers to the percentage away from the nominal voltage rail to trigger
a reset and is shown in Equation 3.
(3)
For example, a 3.3V device (TPS389033) will trigger a reset at 3.17V not 3.3V since 3.3V is considered a
normal operating condition. This means the device tolerance for TPS389033 is ~4%.
11
Mitigating the Indeterminate Output of a Voltage Supervisor (Reset IC) During Power
Up/Down?
Voltage supervisors require a minimum voltage called the Power-on Reset Voltage (Vpor) on the supply
pin before the supervisor is operational. The output voltage of the supervisor is in an indeterminate state
when the voltage supply is below Vpor. By simply connecting a JFET at the output, this indeterminate
voltage can be eliminated.
Please see Mitigating the Indeterminate Output of a Voltage Supervisor (Reset IC) During PowerUp/Down TechNote for a detailed explanation.
12
What is the Minimum Pulse Width to Cause a Reset?
There are typically three ways to trigger a reset on a voltage supervisor/detector:
•
•
•
VDD or SENSE (voltage fault)
Manual Reset
Watchdog
For VDD, the minimum pulse width to cause a reset is specified as "Pulse duration at VDD" and refers to
the minimum pulse width on VDD for the voltage value to be recognized by the device. For devices with
the SENSE pin, the minimum pulse width to cause a reset is specified as "SENSE pin glitch immunity".
For a reset caused by the Manual Reset pin, the minimum pulse width is specifed as "MR pin glitch
immunity". For a reset caused by the watchdog, the pulse width is specified as "Minimum WDI pulse
duration". All of these pulse widths are specified as typical values only with the exception of VDD which is
most often specified as a minimum value.
13
How to Latch a Voltage Supervisor? How to Latch a Watchdog Timer?
Some applications require a latching output whenever a fault occurs that requires user intervention before
the system can resume normal operation. In this case, latching circuitry can be added to Voltage
Supervisors to either latch the RESET, the WDO, or both. Please refer to Latching a Voltage Supervisor
(Reset IC) and Latching a Watchdog Timer TechNotes for a detailed explanation.
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How to Calculate Pull-up Resistor Value for Active-Low, Open-Drain Devices?
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How to Calculate Pull-up Resistor Value for Active-Low, Open-Drain Devices?
To calculate the value of the pull-up resistor for active-low, open-drain devices three specifications need to
be considered. These are the pull-up voltage, the recommended maximum current through the RESET
pin, and VOL. To calculate the resistor, simply use Ohm’s law of V=IR, where V is the difference of Vpull-up
and VOL and I is the current through the pull-up resistor, specifically chosen to satisfy the maximum current
limit of the RESET pin and minimum current required by the load. Please refer to Figure 13
Vs
Vpull-up
Ipull-up
VDD
Voltage Supervisor
(Open-drain output)
Rpull-up
IRESET
ILoad
RESET
Logic
Load
GND
ILoad (minimum) < Ipull-up< IRESET (max)
Figure 13. Calculating Pull-Up Resistor for Open-Drain Outputs
15
How to Add Hysteresis to a Voltage Supervisor Detection Threshold?
Some applications require supply voltage hysteresis, a voltage difference between rising and falling trigger
thresholds, to keep a voltage supervisor from falsely resetting the system due to dips and glitches in the
voltage supply. The TPS3808, for example, inherently has internal sensing voltage hysteresis of 6mV.
That means the rising threshold is 6mv higher than the falling threshold and that amount of hysteresis may
not be enough in some applications to avoid false resets. As shown in Figure 14, without enough
hystereisis, a voltage supply with ripple at the fault trigger threshold can cause the output to continuously
trigger which is undesired.
Figure 14. Voltage Monitoring Without Hysteresis (Left) and With Hysteresis (Right)
The solution to fix this issue is to add hysteresis so that the sense input must rise or fall by a larger
voltage to release or trigger a fault. This is accomplished by adding a hysteresis resistor, Rh, as shown in
Figure 16.
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What are Back-Up Battery Voltage Supervisors and Why are they Needed?
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V1
V2
Rh
R1
Rp
RESET
SENSE
Vs
R2
+
- Vt
Voltage Detector
or Supervisor
Figure 15. Adding Hysteresis Circuit
This effectively prevents AC ripple or voltage glitches on the supply rail to falsely trigger resets.
Please see Adding Hysteresis to Supply Voltage Supervisor.
16
What are Back-Up Battery Voltage Supervisors and Why are they Needed?
Battery back-up voltage supervisors are used in applications that require data retention of CMOS RAM
during fault conditions. When the voltage at Vdd drops below a preset threshold, the reset output asserts
AND Vout switches from Vdd to Vbat so that the battery can directly supply power instead of the falling
voltage on Vdd. Battery back-up voltage supervisors are important in applications where brownout
conditions cannot be tolerated as the back-up battery will provide the system with power and prevent any
system malfunction or loss of data.
Please see Using the TPS3619 with MSP430 Microcontroller Can Reduce the System Power
Consumption with Charge Pumps for more information on other back-up battery voltage supervisor
applications.
17
Summary
For questions not addressed in this document, please visit e2e.ti.com/support/power-management to post
a question. These questions will be answered by a voltage supervisor expert, and may be featured in our
next revision of this FAQ document.
18
References
These resources provide additional information and related documentation for Voltage Supervisors:
• To learn more about voltage supervisor basics, watch Voltage Supervisor Basics Video Series
• To understand voltage supervisor features in more detail, read Selecting Voltage Detectors,
Supervisors, and Reset ICs for System Safety: Part 1 and Selecting Voltage Detectors, Supervisors,
and Reset ICs for System Safety: Part 2
• To see the voltage supervisor options available, see Supervisors and Reset ICs Quick Reference
Guide
• Visit the TI Voltage Supervisors homepage: Overview for Supervisors and Reset ICs
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